1. Field of the Invention
The present invention pertains to the preparation of wood laminates using an improved adhesive. The present invention particularly relates to the preparation of wood laminates using a low molecular weight acetone-formaldehyde resin as a cure rate accelerator for a phenol-formaldehyde adhesive. The present invention permits the preparation of wood laminates from high moisture content veneers.
2. Description of Related Art
Due to their bonding and durability characteristics, phenol-formaldehyde resins have long been favored as the adhesive for preparing wood laminates such as plywoods, particularly wood laminates suitable for exterior use. Typically, these adhesives are applied to the mating surfaces of wood veneers and the veneers then are subjected to a pressing operation at an elevated temperature to consolidate them into a unitary panel and to cure the adhesive. Often, the pressing is performed in two stages to maximize output; using a first or pre-pressing stage at ambient temperature and under a pressure and for a time just sufficient to consolidate the veneers into a unitary panel. The consolidated panel can be stored and handled without shifting or separation of the veneers and afterwards can be treated under heat and pressure to cure the thermosetting adhesive and form the final laminate.
In preparing wood laminates such as plywood, it is conventional to dry the wood veneers to very low moisture content, i.e., to less than about 5% moisture (5 lbs. water/100 lbs. dry wood), before application of the adhesive and consolidation of the veneers. Use of high moisture content veneers in the conventional process invariably leads to a large number of lamination defects and an excessively large number of rejected panels because of poor lamination. The lamination defects are believed to be caused by steam formation between veneer layers and a blow out of the steam when pressure is released upon completion of the pressing cycle. As the temperature increases in the center of the veneers during consolidation, so does the vapor pressure of trapped steam. As the press is opened, the built-up vapor or steam seeks an avenue of escape and blows the panel. Sizable operating and capital costs have been incurred in the prior art to assure consistently low moisture content for wood veneers used in making wood laminates so as to eliminate lamination defects and reduce the number of rejected panels.
Even so, the prior art has recognized the advantages to be gained in both operating and capital costs if higher moisture content veneers could be employed in the preparation of wood laminates. In U.S. Pat. No. 4,239,577, for example, a process is described for preparing wood laminates from high moisture content wood veneers. In accordance with this process, panels are prepared using veneers having different moisture contents with higher moisture content veneers constituting the outer layers of the consolidated panel and lower moisture content veneers constituting the core. While this process purports to ameliorate problems encountered when using high moisture content veneers, it does not eliminate the need for drying at least some of the veneers to low moisture content.
In U.S. Pat. No. 4,441,945, an adhesive is described which purportedly permits the preparation of wood laminates using high moisture content veneers. The adhesive composition combines phenol-aldehyde resin, an alkaline catalyst and from 5-50%, based on the weight of resin solids, of bentonite clay. The adhesive also may contain other conventional additives such as fillers and extenders. Apparently, this adhesive has not received widespread acceptance in the plywood industry, which continues to search for ways to reduce substantially the need to dry high moisture content veneers before assembling them into unitary wood laminates.
Reduced drying requirements result in savings in capital costs and processing time and also lead to improvements in the dimensional stability of the consolidated wood laminants. conventional laminates made with veneers having less than about 5% moisture content tend to swell in size as the moisture content of the wood laminate increase to their equilibrium content gradually with time. In the summer months, the equilibrium moisture content of a wood laminates may be as high as about 10% by wt. Finally, the ability to prepare laminates using high moisture content veneers, e.g. about 10 wt. %, also relaxes the criticality of moisture control in veneer preparation.